In recent years, fine fibers, such as nanowires, nanotubes, and nanohorns, have been studied intensely. As a material forming the nanowires, silver, silicon, gold, copper, zinc oxide, titanium oxide, gallium nitride, or the like have been studied. For example, carbon nanotubes are known as the nanotubes, and carbon nanohorns are known as the nanohorns.
The carbon nanotube that is most promising as an electrically-conductive material is formed such that a graphite sheet is rounded to have a cylindrical shape. Then, the material has a hollow structure having a diameter of about 0.7 to 100 nm and a length of several micrometers to several millimeters. An electrical property of the carbon nanotube depends on the diameter and chirality and shows a metal-like property or a semiconductor-like property. Further, since the carbon nanotube does not have a dangling bond, it is chemically stable. Moreover, since the carbon nanotube is formed only by carbon atoms, it attracts attention as a material which is low in environmental load.
Since the carbon nanotube has the above physical properties, it is expected as an electron emitting source of a flat panel display, an electrode material of a lithium battery, and an electrode material of an electric double layer capacitor, or it is expected to be applied to a probe.
The carbon nanotube may be synthesized by arc discharge using a carbon electrode, thermal decomposition of benzene, laser deposition, or the like. However, graphite is synthesized in addition to the carbon nanotube by these methods. Therefore, in the case of applying the carbon nanotube to the electron source, the electrode of the battery, the probe, or the like, impurities, such as graphite and carbon nanoparticles, need to be removed in advance. Moreover, since the carbon nanotubes having various lengths are synthesized in random directions, the properties thereof as the electron emitting source are limited.
In recent years, a method for directly synthesizing oriented carbon nanotubes was presented. For example, a method for obtaining single-wall nanotubes densely and vertically oriented on a Si wafer by using plasma CVD has been developed. In accordance with this method, it is possible to obtain the carbon nanotubes in which the impurities, such as the graphite and the carbon nanoparticles, are small in amount and the directions of the fibers are the same as one another. With this, the manufactured carbon nanotubes are easily applied to the electron sources, the electrodes of the batteries, the probes, and the like.
Moreover, research development for applying the carbon nanotube to the electrode of the electric double layer capacitor by utilizing the size of a surface area has been intensely studied. Further, there exists an example in which the carbon nanotubes grown vertically on the surface of a current collector using the above-described technology are used as the electrodes.
The electric double layer capacitor is a condenser utilizing an electric double layer formed between an active material and an electrolytic solution. The electric double layer capacitor has been used as a backup power supply, and it is recently adopted in electric cars. Thus, future rapid growth of the electric double layer capacitor is expected. As a conventional active material of the electric double layer capacitor, activated carbon is widely known (see PTL 1, for example). However, since the carbon nanotube has much larger external surface area of 2,600 to 3,000 m2/g than the activated carbon and has, for example, an extremely strong mechanical property and an excellent electronic property, the electric double layer capacitor using the carbon nanotube as the active material is attracting attention.
The electric double layer capacitor is different in operating principle from a battery utilizing an oxidation-reduction reaction. The electric double layer capacitor is a power storing device configured to be charged and discharged by the adsorption and desorption of the positive ions and negative ions of an electrolytic solution onto the surface of the active material. Since the electric double layer capacitor is free of chemical reactions, it is more excellent than the battery. For example, the electric double layer capacitor has a long life, measurement of remaining electric charge is easy, and the environmental load is low.
FIG. 11 shows one example of the configuration of a common electric double layer capacitor in order to explain an electric operating principle. An electric double layer capacitor 1100 includes a positive electrode 1111 and a negative electrode 1112. The positive electrode 1111 includes a current collector 1107 and an active material layer 1108 formed on the current collector. The negative electrode 1112 includes a current collector 1104 and an active material layer 1105 formed on the current collector. The positive electrode 1111 and the negative electrode 1112 are provided in an electrolytic solution 1106. By applying a voltage to each of the positive electrode 1111 and the negative electrode 1112 by a power supply 1101, an electric field is generated between the positive electrode 1111 and the negative electrode 1112. By the effect of the electric field, positive electric charge 1109 is generated inside the active material layer 1108 of the positive electrode 1111, so that negative ions 1110 are attracted to the positive electrode 1111. Moreover, negative electric charge 1103 is generated inside the active material layer 1105 of the negative electrode 1112, so that positive ions 1102 are attracted to the negative electrode 1112. Thus, electricity is stored. The electric field generated between the positive electrode 1111 and the negative electrode 1112 by the voltage application disappears by the adsorption of the negative ions 1110 onto the positive electrode 1111 and the adsorption of the positive ions 1102 onto the negative electrode 1112. Instead of this, electric double layers are respectively generated between the positive electrode 1111 and the negative ions 1110 and between the negative electrode 1112 and the positive ions 1102. The total of a potential difference between these two electric double layers becomes a potential difference between the electrodes.
An electrical equivalent circuit corresponding to FIG. 11 is shown in FIG. 12. As shown in FIG. 12, an electric double layer capacitor is configured such that two condensers 1202 and 1203 are serially connected to each other.
Electric charge Q stored in the condenser is commonly shown by Q=CV, where C denotes the capacitance of the condenser and V denotes the voltage difference. An energy E stored in the condenser is shown by E=½CV2. Therefore, stored energy per unit volume of the active material is proportional to the capacitance of the condenser per unit volume of the active material. On this account, by increasing the density of the active material layer without decreasing the area where the ions adsorb, the stored energy per unit volume of the active material can be increased.
NPL 1 reports a carbon nanotube densification technology which achieves more than 0.6 g/cm3 by a simple high-pressure press (1 to 10 t/cm2 (98 to 980 MPa)) without almost decreasing the surface area and the electric capacitance.